vmm: Unify implementations of vcpu_cleanup()

Different vmm implementations were freeing the per-vCPU structure in
different places. Make the implementations consistent. No functional
change intended.

Reviewed by: corvink
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53010